Cosmology is a branch of astronomy involving the origin, structure and evolution of the Universe as a whole. It deals with phenomena at a scale of several million light-years. It is derived from the Greek 'kosmos', meaning 'order' and 'everything' and 'logos', meaning 'word' or 'discourse'. To understand the physical universe as a unified whole, it involves natural sciences, such as astronomy and physics.
There is a lot more universe beyond what we can observe, with light from most of the galaxies not having had time to reach us. The Universe is possibly infinite. The separation between stars is so large that we measure their distances in units of how far light can travel in a given amount of time (see What is a light year?)
We see the Sun as it looked 8 minutes ago, as it is 500 light-seconds from Earth, but even our next closest star, Proxima Centauri, is 4.3 light-years (ly) away. The stars on the opposite side of the Milky Way from the Sun are around 100 000 ly away and this is a typical diameter of a large spiral galaxy. Our nearest galaxies are the Large and Small Magellanic Clouds, which are relatively small (containing around 10⁹ stars) and are about 200 000 ly away.
The Cosmos is growing at an ever-increasing rate in what is called cosmological expansion. Galaxies are travelling away from each other with an apparent velocity of recession that is directly proportional to the distance of the object. This relationship is known as Hubble's Law, after its discoverer Edwin Powell Hubble. 13.8 billion years ago all the Universe's matter was closely packed together in an extremely dense state, until a 'big bang' explosion dispersed it.
Cosmologists estimate the oldest photons we can observe have travelled 45-47 billion ly since the Universe began. So the observable Universe is 93 billion ly wide.
Albert Einstein showed that space has structure - it is flexible and can be stretched.
Expansion of the Universe is actually the stretching of space between clusters of galaxies, not the motion of galaxies through space. Space may possess energy of its own of a form previously not known.
One of the challenges in cosmology is whether space and time are infinite or finite and despite centuries of study we have still not arrived at a conclusion. So cosmology studies the potential end of the Universe.
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